The present invention relates to a novel oxygen-generating electrode and a method for the preparation thereof. More particularly, the invention relates to an electrode having excellent durability and low oxygen overvoltage for generating oxygen by electrolytically oxidizing an aqueous solution on an anode as well as to a method for the preparation thereof.
A type of conventional metallic electrodes widely used in the electrolytic industry includes those prepared by providing an overcoating layer of a platinum group metal or an oxide thereof on an electroconductive substrate made from titanium metal.
For example, known electrodes used as the anode for producing chlorine by the electrolysis of brine include those prepared by providing a titanium substrate with an overcoating layer formed of an oxide mixture of ruthenium and titanium or an oxide mixture of ruthenium and tin (see, for example, Japanese Patent Publications 46-21884, 48-3954 and 50-11330).
Besides the above mentioned process of electrolysis of brine in which chlorine is produced as the electrolytic product. various processes are known in the electrolytic industry in which oxygen is generated on the electrode. Examples of such an oxygen-generating electrolytic process include recovery of spent acids, alkalis or salts, electrolytic metallurgy of copper, zinc, etc., metal plating, cathodic protection and the like.
These oxygen-generating electrolytic processes require electrodes quite different from the electrodes successfully used in the electrolytic processes accompanied by generation of chlorine. When an electrode for the chlorine-generating electrolysis, such as the above mentioned titanium-based electrode having a coating layer of an oxide mixture of ruthenium and titanium or ruthenium and tin, is used in an oxygen-generating electrolytic process, the electrolysis must be discontinued before long due to rapid corrosion of the electrode. Namely, the electrodes must be specialized for the particular electrolytic processes. The electrodes most widely used in an oxygen-generating electrolysis are lead-based electrodes and soluble zinc anodes although other known and usable electrodes include iridium oxide- and platinum-based electrodes, iridium oxide- and tin oxide-based electrodes, platinum-plated titanium electrodes and the like.
These conventional electrodes are not always quite satisfactory due to the troubles which may be caused depending on the type of the oxygen-generating electrolytic process. When a soluble zinc anode is used in zinc plating, for example, the anode is consumed so rapidly that adjustment of the electrode distance must be performed frequently. When a lead-based insoluble electrode is used for the same purpose, a small amount of lead in the electrode is dissolved in the electrolyte solution to affect the quality of the plating layer. Platinum-plated titanium electrodes are also subject to rapid consumption when used in a process of a so-called high-speed zinc plating process at a high current density of 100 A/dm.sup.2 or higher.
Accordingly, it is an important technical problem in the technology of electrode manufacture to develop an electrode useful in an oxygen-generating electrolytic process which can be used with versatility in various processes without the above mentioned drawbacks.
When an oxygen-generating electrolytic process is performed by using a titanium-based electrode having a coating layer thereon, on the other hand, it is not rare or rather usual that an intermediate layer of titanium oxide is formed between the substrate surface and the coating layer to cause a gradual increase in the anode potential or eventually to cause falling of the coating layer with the substrate surface being in a passive state. Various attempts and proposals have been made to provide an appropriate intermediate layer beforehand between the substrate surface and the coating layer in order to prevent subsequent formation of a layer of titanium oxide (see for example, Japanese Patent Publications 60-21232 and 60-22074 and Japanese Patent Kokai 57-116786 and 60-184690).
The electrode having an intermediate layer provided as mentioned above is not so effective as desired when the electrode is used in an electrolytic process at a high current density because the electroconductivity of such an intermediate layer is usually lower than the overcoating layer.
It is also proposed to provide an intermediate layer formed by dispersing platinum in a matrix of a non-precious metal oxide (see Japanese Patent Kokai 60-184691) or to Provide an intermediate layer formed of an oxide of a valve metal, e.g., titanium, zirconium, tantalum and niobium, and a precious metal (see Japanese Patent Kokai 57-73193). These electrodes are also not quite advantageous because platinum has no very high corrosion resistance in itself in the former type and, in the latter type, the kind of the valve metal oxide and the compounding amount thereof are not without inherent limitations.
Besides, Japanese Patent Kokai 56-123388 and 56-123389 disclose an electrode having an undercoating layer containing iridium oxide and tantalum oxide on an electroconductive metal substrate and an overcoating layer of lead dioxide. The undercoating layer in this electrode, however, serves to merely improve the adhesion between the substrate surface and the overcoating layer of lead dioxide to exhibit some effectiveness to prevent corrosion due to pinholes. When such an electrode is used in an oxygen-generating electrolytic process, disadvantages are caused because of the insufficient effect of preventing formation of titanium oxide and unavoidable contamination of the electrolyte solution with lead.
The inventors have previously proposed an improved oxygen-generating electrode of which the electroconductive substrate of, for example, titanium metal is provided with an undercoating layer compositely consisting of iridium oxide and tantalum oxide in a specific molar proportion and an overcoating layer of iridium oxide formed thereon (see, Japanese Patent Kokai 63-235493). The electrode of this type having a bilayered coating, however, is not quite satisfactory in respect of the oxygen overvoltage which cannot be low enough to be desirably 400 mV or lower although an improvement can be obtained in the durability of the electrode. Further, the inventors have proposed an electrode having a ternary composite coating layer of iridium oxide, tantalum oxide and platinum metal formed on an electroconductive substrate in a specific molar proportion (see Japanese Patent Kokai 1-301876). The performance of the electrode of this type is indeed superior to the above described electrode with a bilayered coating and satisfactory if it is not for the expensiveness of the platinum metal.